thumbnail support on twentyten

// Add theme support for post thumbnails
add_theme_support('post-thumbnails');
add_image_size( 'custom_thumb', 150, 150, true);
 

// Add thumbnail to excerpt
add_filter( 'the_excerpt', 'excerpt_thumbnail' );
function excerpt_thumbnail($excerpt){
    if(is_single()) return $excerpt;
    global $post;
    if ( has_post_thumbnail() ) {
        $img .= ''.get_the_post_thumbnail($post->ID, 'custom_thumb').'';
    } else {
        $img = '';
    }
    return $img.$excerpt;
}